Hammer drills are known in which rotation of toothed surfaces against each other causing a hammering action. Also, in U.S. Pat. Nos. 3,149,681 and 3,133,602, rotary impact hammers with a ball on tooth engagement provide for a hammering action only in one direction of rotation. A ball on tooth engagement also tends to wear a groove in the tooth, which tends to create a wide contact area between ball and tooth. Together with the immobility of the tooth surface, the wide contact area increases friction losses and heating of the tool. A further hammer drill is disclosed in U.S. Pat. No. 6,684,964, in which the hammer action is provided by impact of facing sets of bearings. This design suffers from increased wear and friction losses from the impact on the bearings on each other.